The hydration of propylene to isopropyl alcohol in sulfuric acid solution through a two-step esterification-hydrolysis process was one of the first petrochemical processes, dating back to the early 1920's.
The liquid-phase, two-step process has, however, several serious disadvantages, such as the corrosive nature of the 85% sulfuric acid necessary for the propylene esterification reaction; the necessity of diluting the acid reaction medium to promote hydrolysis and to facilitate separation of the acid from the alcohol after hydrolysis and the necessity of acid reconcentration before recycle to the esterification step of the process; and the frequent need for neuralizing the alcohol product. Consequently, the direct hydration of propylene over solid catalysts is of practical significance to overcome the above difficulties.
Considerable work has therefore been done on the development of propylene hydration scheme, which do not use sulfuric or other liquid acids. The processes which have been studied generally involve contacting the olefin and water in the presence of a suitable solid catalyst at elevated temperature and pressure to produce the desired isopropyl alcohol directly. High pressures, relatively low temperatures and high steam to propylene ratios favor conversion to the alcohol.
When the hydration catalyst is active at low temperatures, the reaction is usually conducted in the presence of a large excess of liquid water (thus under corresponding high pressure). Kaiser, Beuther, Moore and Odioso (Ind. Eng. Chemistry Product Res. and Development Vol. 1 pp 296-302 (1962)) using sulfonated styrenedivinylbenzene ion-exchange resins (such as Rohm and Haas Co. Amberlyst 15 or IR-120) studies the effects of temperature, pressure, space velocity and feed composition on conversion and selectivity. When the catalyst used is not active at lower temperatures (&lt;150.degree. C.) vapor-phase operation is used to avoid extreme pressures. The thermodynamic equilibria prevailing at higher temperatures limits the propylene conversion to quite low levels.
Some of the reported hydration processes used supported mineral or inorganic acids in a relatively low pressure, essentially vapor phase process for propylene hydration.
Catalysts used include silicophosphoric acid (U.S. Pat. No. 2,876,266) phosphoric acid on celite (U.S. Pat. No. 2,579,601) and tungstic acid on alumina (F. J. Sanders and B. F. Dodge, Ind. Eng. Chem., 26, 208 (1934).
The conversion reported for vapor-phase operations such as over phosphoric acid impregnated on Celite brand of diatomaceous earth at 225.degree.-250.degree. C., 550 psig is only 3.8%.
The vapor phase, direct hydration processes eliminate the major difficulties of the two-step sulfuric acid based solution process but, so far, had the disadvantage of low per-pass conversion, which is considered to be the result of thermodynamic vapor phase equilibrium conditions. Work, consequently, was directed to the use of conditions, which permit reactants to be both in the vapor (propylene) and liquid (water) phases.
Processing with liquid water permits the direct hydration of olefins in much higher per-pass conversions than can be obtained by vapor phase operation, probably because the solubility of the product alcohol in the liquid phase water changes the thermodynamic equilibrium consideration which limit the vapor phase conversions. The catalysts which can be utilized in this mixed phase processing must then show hydrothermal stability in the presence of liquid phase water at the required reaction temperature, as well as catalytic activity. The most commonly utilized catalyst for this process known until now was tungsten oxide, although silica-alumina and supported Group VI and Group VII metals also have been reported.
Ogino ((Shokubai, Tokyo) 4, 73 (1962) J. CATAL., 8, 64 (1967)) found metal sulfate-silica gel catalysts, particularly those of Fe, Al, Cr, Cu, Zn, Co, Pd, Ni, Mn, and the like, active for the hydration of propylene. He concluded that with these catalysts, an acidity range of H.sub.o of 1.5 to -3 is the most suitable. However, methods used in establishing surface acidities of the catalysts based on color changes of indicator dyes must be considered unreliable. There was, thus, no clear indication available as to what to expect with high acidity solid catalysts, particularly with solid superacids.
Sulfonic cation exchange resins were also claimed as catalysts. Japan Kokai No. 77,151,106 (Chemical Abstracts 88, 169578x) discloses the use of halogenated cation exchange resins of the sulfonic acid type for the hydration of lower olefins to form the corresponding alcohols. Example 1 of this patent reports the hydration of propylene to form isopropyl alcohol over a chlorinated Amberlyst 15 sulfonated styrene-divinylbenzene copolymer. The conversion rate of propylene was reported as 14.3% and the selectivity of isopropanol as 96%. A temperature of 130.degree. C. and a pressure of 70 atm (about 1000 psi) was needed for the reaction. The same patent also claims the use of Du Pont Nafion 501 resin, a "perfluoro vinyl ether copolymer containing sulfonic radical", as catalyst.
Example 2 describes the hydration of butene-1 with this catalyst to sec.-butanol with a 3.1% conversion. The very low conversion is not explained, but its reason is apparent when considering that the commercial Nafion 501 resin used is the potassium salt of the resin sulfonic acid and not an acid catalyst. Clearly the commercial Nafion resin, which is the potassium salt of the sulfonic acid resin used in ion-membrane applications, is unsuitable as a hydration catalyst.